1. Field of the Invention
This invention relates to rechargeable batteries, and particularly to charging rechargeable lithium-ion (Li-ion) batteries.
2. Description of the Related Art
Rechargeable batteries are used in a variety of electronic devices, including portable computers, portable computer peripherals, personal digital assistants (PDAs), cellular phones, and cameras. Because of the wide variety of uses for rechargeable batteries, a number of different rechargeable battery chemistries have been developed, each having certain advantages and disadvantages. Among the most commonly used battery chemistries are: nickel cadmium (NiCd), nickel-metal hydride (NiMH), lithium ion (Li-ion) and lithium-polymer (Li-polymer).
NiCd batteries have nickel and cadmium electrodes and a potassium hydroxide electrolyte. NiCd batteries are the most common rechargeable batteries, however, they are subject to a number of problems. For example, NiCd batteries have a memory effect, which is a loss of battery capacity caused by recharging the battery before it is fully discharged. Additionally, NiCd batteries are susceptible to over-charging, which causes the battery to develop internal short circuits, thereby causing the battery to run down prematurely which may eventually cause the battery to take no charge at all. Additionally, cadmium is a poisonous heavy metal, and so properly disposing of NiCd batteries requires great care and considerable expense.
NiMH batteries offer higher energy density than NiCd batteries, eliminate many of the disposal problems, and are relatively inexpensive. NiMH batteries have a hydrogen-absorbing alloy anode, a nickel compound cathode, and a potassium hydroxide electrolyte. However, NiMH batteries also have a number of disadvantages. For example, NiMH batteries have a high self-discharge rate, are subject to voltage depression (an effect similar to the memory effect seen in NiCd batteries), and are sensitive to thermal conditions.
In recent years, Li-ion batteries have become the rechargeable battery of choice in devices such as portable computers. The chemistry behind Li-ion batteries involves lithium-plated foil anodes, an organic electrolyte, and lithium compounds within carbon electrodes. Li-ion batteries have very high energy densities (e.g. at least twice that of NiCd batteries), better cycle life than NiMH or NiCd batteries, higher output voltages, and lower self-discharge rates.
There are at least two basic types of Li-ion batteries in use today: coke electrode batteries and graphite electrode batteries. While coke electrode Li-ion batteries represent a more mature technology, batteries using graphite electrodes provide a flatter discharge curve, that is their output voltage tends to decrease more slowly, particularly in the later stages of battery discharge. Other Li-ion systems are also being developed, including batteries that use a carbon electrode (graphite or coke) along with an amorphous tin-based composite oxide material electrode.
One thing that all of these Li-ion battery systems have in common is the need for careful monitoring and control during charging and discharging. For safety and longevity reasons, Li-ion battery packs are equipped with control circuitry to limit the maximum voltage of each cell in the pack during charge, and to prevent each cell's voltage from dropping too low on discharge. Additionally, the control circuitry limits maximum charge current and discharge current, and monitors the temperature of the battery pack cells. All of these monitoring steps help to reduce or eliminate the possibility of metallic lithium plating on the electrodes, which can cause internal shorts, thermal runaway, and ultimately a violent reaction in the battery.
In addition to the safety concerns, schemes for charging Li-ion batteries should also take into account the desire to charge batteries as quickly as possible, to prolong the lifetime of batteries (e.g., maximize the number of charge-discharge cycles), and to charge batteries to full capacity. FIG. 1 illustrates a known method of charging Li-ion batteries referred to as constant-current/constant-voltage (CC/CV) charging. Curve 100 represents the battery voltage over the duration of the charge, while curve 110 indicates the battery current over the same period. Constant current is supplied to the battery (in the case of FIG. 1, the charging characteristics are shown for a two-cell battery pack) until the cell voltage reaches a threshold voltage. In FIG. 1, the threshold voltage is approximately 4.2V per cell. The cell threshold voltage largely depends upon the manufacturing details of the cell, and is generally 4.1V for graphite electrode Li-ion cells, and 4.2V for coke electrode cells. Both types of Li-ion cell typically have a threshold voltage tolerance of .+-.0.05V. Newer types of Li-ion cells may have threshold voltages and tolerances different from those for coke and graphite electrode cells. Once the threshold voltage has been reached, charging changes to a constant voltage mode where the battery is charged at or near the threshold voltage, but the current gradually (and naturally) decreases over time. A variety of different charge termination techniques can be used to end the charging cycle. While CC/CV charging is appealing because of its simplicity, in practice there are a number of drawbacks associated with the technique, including the length of charge time generally required, and increases in charge time caused by changes in cell impedance that occur over time.
One method for overcoming some of the limitations of CC/CV charging is illustrated in FIG. 2. Charging begins with a constant current phase 210 that leads to a corresponding rise in battery voltage 200, just as in the constant current phase shown in FIG. 1. However, instead of switching over to a constant voltage mode when the battery voltage reaches the threshold voltage 205, the charger provides a series of constant current pulses 220. Note that voltage threshold 205, can be the voltage threshold specified by the manufacturer, or a voltage value lower than the manufacturer's specified voltage threshold (e.g., for additional safety). Application of charging pulses begins after the battery voltage has fallen back to or below the threshold voltage. As illustrated, each pulse 220 has a constant duration. Alternately, pulse charging systems begin applying the constant current, and then start a timer when the battery voltage meets or just exceeds the threshold voltage. When the timer reaches a predetermined count, the current pulse is shut off. In either case, the length of the pulse is chosen so that the battery voltage does not exceed the known tolerance of the threshold voltage, or at least does not exceed the threshold for too great a period. Once the battery voltage falls back down to or below the threshold voltage, the process is repeated. As charging progresses, the time required for the battery voltage to fall back to, or below, the threshold voltage (often called the relaxation time) increases. Consequently, the time between current pulses, such as time periods 230 and 240, increases. Initially, the "off" times such as 230 and 240 are short, on the order 1 ms, but as the battery's state of charge increases, the "off" times can increase to tens of seconds, minutes, and eventually hours.
While pulse charging can decrease the charge time as compared to CC/CV charging, the technique is not without its own limitations. As noted above, in at least one pulse charging scheme, timing of the pulse begins only after the battery voltage meets or exceeds the threshold voltage. However, applying a constant current pulse at the same current level used during constant-current charging can cause the battery voltage to reach the threshold voltage too quickly, or to spike, that is, briefly increase beyond the threshold voltage and then return back to or below the threshold voltage. Such spiking can be caused by, for example, the changes in the internal impedance of the battery cells. However, in either case, the pulse timer starts sooner than is desired, thereby prolonging the overall charge time.
Accordingly, it is desirable to reduce the total charge time for a rechargeable battery and to increase the length of the time periods when current pulses are on in a pulse charging system. Additionally, it is desirable accomplish these goals while continuing to adequately protect the rechargeable battery and operate it in an efficient manner.